Introduction: The aim of this study was to examine the associations of elevated serum C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) with change in muscle strength in patients with established knee osteoarthritis (OA), at 2 years.Methods: Data from 186 patients with knee OA were gathered at baseline and at 2-year follow-up. CRP (in milligrams per liter) and ESR (in millimeters per hour) were measured in serum from patients' blood. Strength of quadriceps and hamstrings muscles was assessed by using an isokinetic dynamometer. The association of inflammatory markers with change in knee muscle strength was analyzed by using uni- and multi-variate linear regression models.Results: Patients with elevated CRP values at both baseline and 2-year follow-up exhibited a lower increase in knee muscle strength for a period of 2 years (β = -0.22; P = 0.01) compared with the group with non-elevated levels at both times of assessment. The association persisted after adjustment for relevant confounders. Elevated ESR values at both times of assessment were not significantly associated with change in knee muscle strength (β = -0.05; P = 0.49).Conclusions: Our results indicate that elevated CRP values are related to a lower gain in muscle strength over time in patients with established knee OA. Although the mechanism to explain this relationship is not fully elucidated, these results suggest inflammation as a relevant factor influencing muscle strength in this group of patients. © 2014 Sanchez-Ramirez et al.; licensee BioMed Central Ltd.